Method for the production of lignin-containing precursor fibres and also carbon fibres

ABSTRACT

The invention relates to a method for the production of a precursor for the production of carbon- and activated carbon fibres according to the wet- or air-gap spinning method, in which a solution of lignin and a fibre-forming polymer in a suitable solvent is extruded through the holes of a spinning nozzle into a coagulation bath, the formed thread is stretched and subsequently treated, dried at an elevated temperature and then wound up. The lignin-containing thread is an economical starting material for the production of carbon- and activated carbon fibres.

The invention relates to a method for the production of a precursor for the production of carbon- and activated carbon fibres according to the wet- or air-gap spinning method, in which a solution of lignin and a fibre-forming polymer in a suitable solvent is extruded through the holes of a spinning nozzle into a coagulation bath, the formed thread is stretched and subsequently treated, dried at an elevated temperature and then wound up. The lignin-containing thread is an economical starting material for the production of carbon- and activated carbon fibres.

Carbon fibres are high-performance reinforcing fibres which are used essentially for composite materials in aircraft construction, high-performance vehicle construction (Formula I, high-performance sailing ships etc.), for sports equipment and increasingly for wind energy plants. Currently, extensive efforts are being made worldwide to introduce carbon fibres of average quality (at a reduced price level) into mass applications in automobile construction, the development of weight-reduced electric vehicles which is sought publicly in the meantime representing an essential driving force.

Carbon fibres are produced by heat treatment above 1,000° C. of organic precursor fibres. The first carbon fibres were developed on the basis of cellulose precursors and used as filaments in lamps. Nowadays, polyacrylonitrile or copolymers of polyacrylonitrile are the dominating polymers for the production of precursors for carbon fibres. The palette of carbon fibres based on PAN is supplemented by high-modulus carbon fibre made from pitch. For PAN-based carbon fibres, the estimated annual capacity in 2010 was at approx. 77,000 t and, for pitch-based carbon fibres, at 1,830 t (technical textiles March 2010). An overview of the production, the structure, the mechanical properties and also the application of PAN- and pitch-based carbon fibres is provided in: J. P. Donnet et al., Carbon fibers, third edition, Marcel Dekker, Inc. New York, Basle, Hong Kong.

Polyacrylonitrile and pitch are products of the petrochemical industry and hence are subject to the typical cost increase for this branch of industry. In the last few years, a development trend for developing precursors which are not linked to the oil price with respect to the raw materials resulted therefrom. This trend was forced by the demand for carbon fibres in the average quality segment and hence also in the average price segment for mass applications, as are seen in automobile construction.

Also biopolymers thereby came into focus. Reference was made already to cellulose (rayon) as raw material for the first carbon fibre. Also Lyocell fibres were examined as precursor (S. Peng et al., J. Appl. Polymer Sci. 90 (2003) 1941-1947). It was shown that the Lyocell-based carbon fibres have somewhat greater strength than the rayon-based fibres produced under comparable conditions. 1 GPa for the strength and approx. 100 GPa for the modulus of elasticity are however at a very low level for carbon fibres. In addition to the cellulose man-made fibre, also cellulose natural fibres were tested as precursor for carbon fibre. M. Zhang et al. (Die Angewandte makromolekulare Chemie (Applied macromolecular chemistry) 222 (1994) 147-163) used sisal fibre as precursor for carbon fibre production. With a strength of 0.82 GPa and a modulus of elastocity of 25 GPa, the carbon fibres produced therefrom are at a very low level.

Another biopolymer which increasingly is gaining importance in precursor development is lignin. Lignin is a polyaromatic polyol which is a component of wood and occurs in large quantities as a by-product of cellulose production. The carbon proportion is at approx. 60-65%. The chemical structure of lignin is determined by the type of wood used in the cellulose process and also the method of cellulose digestion which is applied. The main quantities of the resulting lignin are supplied for energy use. With lignin, an extremely economical raw material is available in very large quantities and is in practice not fibre-forming in the unmodified form. One objective was the development of melt-spun lignin-containing precursors. J. F. Kadla et al. (Carbon 40 (2002) 2913-2920) describe the production of lignin fibre by melt-spinning of a commercially available kraft lignin and also melt-spinning of a mixture of lignin with low proportions up to 5% of polyethylene oxide (PEO). Processing of pure lignin requires a thermal pre-treatment which increases the raw material costs and, in mixtures, only small proportions of PEO are possible since, with larger quantities of PEO, adhesion occurs in the stabilising process. The carbon fibres made from the melt-spun lignin-containing precursors had strengths of approx. 0.4 GPa and moduli in the range 40-50 GPa and hence still do not fulfil the mechanical characteristic values sought by automobile construction, strength approx. 1.7 GPa and modulus approx. 170 GPa.

Kubo et al. (Carbon 36 (1998) 1119-1124) describe a process for the melt-spinning of lignin, in which, in a pretreatment step, the non-melting high-molecular components are removed from the lignin. In a further publication, K. Sudo et al. (J. Appl. Polymer Sci., 44 (1992) 127-134) describe the pretreatment of lignin with organic solvents with subsequent melt-spinning of the chloroform-soluble fraction. The carbon fibres produced therefrom had merely a low strength level.

U.S. Pat. No. 7,678,358 claims acetylation of lignin as precursor of lignin melt-spinning without however giving any information relating to the properties of the carbon fibres produced in this way. The state of the art shows that it is possible in principle to produce melt-spun lignin-containing precursors for carbon fibres. However it is also shown that the property level of carbon fibre based on PAN or pitch is not achieved. The question remains open as to whether the lignin modification required to make this suitable for melt-spinning does not again offset the cost advantage of the economical raw material, lignin.

The object underlying the invention is to develop an economical method for the production of a lignin-containing precursor based on a solution-spinning method for the production of carbon- and activated carbon fibres.

Furthermore, it is the object of the present invention to indicate a corresponding lignin-containing precursor fibre. In addition, the present invention relates to further processing of the precursor fibres to form carbon fibres and also correspondingly produced activated carbon fibres.

This object is achieved with respect to the method for the production of a lignin-containing precursor fibre by the features of patent claim 1. Patent claim 14 relates to a correspondingly produced precursor fibre. In addition, a method for the production of a carbon fibre is indicated by patent claim 16, with patent claim 18 a correspondingly produced carbon fibre is provided.

In the case of the method according to the invention for the production of a lignin-containing precursor fibre for the production of carbon fibres and/or activated carbon fibres, a solution, comprising at least one sort of lignin and also at least one fibre-forming polymer selected form the group consisting of cellulose or cellulose derivates in at least one solvent, selected from the group consisting of tertiary amine oxides, ionic Liquids, aprotic polar solvents, dimethyl formamide and/or dimethyl accetamide is introduced into a coagulation bath by extrusion of the solution through a spinning nozzle by the wet spinning or air-gap spinning method, the lignin-containing precursor fibre precipitating.

In the method according to the invention, the preferably low processing temperature of the solution, once produced, during extrusion thereof into the coagulation bath is particularly advantageous, the maximum upper limit of this temperature being prescribed by the nature of the coagulation bath (boiling point). Generally, the temperature of the coagulation bath is hence below 100° C. As a result, extremely gentle processing of the lignin fibres is provided, which surprisingly leads to the carbon fibres produced from these precursor fibres having significantly increased tensile strength.

There is understood, according to the invention, by the term “solution” that all the components of the solution, i.e. both the lignin and the fibre-forming polymer, are completely solvated by the solvent. However, this term likewise also includes the possibility that the lignin fibres and/or the fibre-forming polymer are present partly undissolved herein.

A preferred embodiment of the method provides that the solution used in step a) is produced by agitation or kneading of the at least one sort of lignin and also of the at least one fibre-forming polymer in the at least one solvent, preferably at temperatures of 60° C. or more, particularly preferred of 80° C. or more.

Furthermore, it is advantageous if the solution is filtered before introduction into the coagulation bath, any possibly contained insoluble components being able to be separated.

In a particularly preferred embodiment, the spinning hole diameter of the spinning nozzle is from 50 to 600 μm, preferably 100 to 500 μm.

The methods for shaping the solution and transferring it into the precipitation bath or coagulation bath are thereby effected in the wet-spinning method or in the air-gap spinning method, the air gap in the case of an air-gap spinning method being preferably at least 10 mm, further preferred at least 20 mm and at most 500 mm.

Further advantageous aspects of the method according to the invention provide that the lignin-containing fibre according to step b)

a) is stretched, preferably is stretched to at least 1.1 times, further preferred to 1.1 to 12 times, particularly preferred to at least 1.5 times, particularly preferred to at least 2 times its length, in particular at a temperature of at least 60° C., preferably at least 80° C., further preferred at least 90° C., particularly preferred of at least 100° C., the stretching being implemented preferably in the precipitation bath, in air or in water vapour,

b) is washed, preferably washed with demineralised water,

c) is treated subsequently with textile aids for improving the thread strength and for avoiding electrostatic charges,

d) is dried, in particular by winding up or winding round of the fibre onto or around heated rollers and/or by through-flow drying at a temperature of at least 80° C., preferably at least 100° C. and/or

e) is wound up.

Furthermore, it is advantageously possible that the fibre is treated with a spinning oil before drying, after drying or before and after drying.

Preferred concentrations of the at least one sort of lignin are thereby from 1 to 99% by weight, preferably from 2 to 30% by weight, particularly preferred from 3 to 20% by weight, relative to the entire solution.

Advantageous concentrations of the at least fibre-forming polymer are thereby, relative to the entire solution, from 1 to 99% by weight, preferably from 5 to 40% by weight, particularly preferred from 7 to 30% by weight, likewise relative to the entire solution.

In a particularly preferred embodiment, the coagulation bath comprises water or a mixture of water and an organic liquid, such as aprotic polar solvents, in particular dimethylsulfoxid (DMSO), such as aliphatic amides which are liquid at room temperature, in particular dimethylformamide (DMF) or dimethylacetamide (DMAc); tertiary amine oxides, in particular N-methylmorpholine-N-oxide; ionic liquids, preferably ionic liquids selected from the group consisting of imidazolium compounds, pyridinium compounds or tetraalkylammonium compounds, particularly preferred 1-butyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium acetate, 1-ethyl-3-methylimidazolium acetate and/or mixtures hereof.

An advantageous pH value of the coagulation bath is thereby between 1 and 7, preferably between 2 and 5.

The solvent for the spinning solution, i.e. the solution which comprises the lignin and also at least one fibre-forming polymer, is thereby selected preferably from the group consisting of aprotic polar solvents in particular dimethylsulfoxide (DMSO) dimethylformamide (DMF) or dimethylacetamide (DMAc); tertiary amine oxides, in particular aqueous N-methylmorpholine-N-oxide (NMMNO, in particular NMMNO monohydrate); ionic liquids, preferably ionic liquids selected from the group consisting of imidazolium compounds, pyridinium compounds or tetraalkylammonium compounds, particularly preferred 1-butyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium acetate, 1-ethyl-3-methylimidazolium acetate; and/or mixtures hereof

The fibre-forming polymers are selected from the group consisting of cellulose and/or cellulose derivatives, in particular cellulose carbamate, cellulose allophanate and hemicellulose and/or mixtures or blends hereof

Furthermore, it is advantageous in the case of the method according to the invention if the at least one sort of lignin results from wood and annual plant pulping methods and is selected in particular from the group consisting of alkali lignin, kraft lignin, lignosulphonate, thiolignin, organosolvlignin, ASAM lignin, lignins from digestion processes by means of ionic liquids or enzymes and/or combinations or mixtures hereof.

According to the invention, a precursor fibre for the production of carbon fibres is likewise indicated. The precursor fibre according to the invention is distinguished by

a) a content of at least one sort of lignin of 1% to 99% by weight, preferably 20 to 60% by weight,

b) a content of at least one sort of fibre-forming polymer of 1% to 99% by weight, preferably 40 to 80% by weight, and

c) possibly a strength of at least 10 cN/tex, preferably at least 20 cN/tex, and

d) possibly a modulus of elasticity of at least 1,000 cN/tex, preferably at least 1,300 cN/tex.

The precursor fibre according to the invention can be produced particularly advantageously according to a previously described method.

Concerning the definitions of lignin and the fibre forming polymers we refer to the method claims 12 and 13 and the description.

Furthermore, a method is indicated according to the present invention for the production of a carbon fibre, in which a precursor fibre according to one of the two preceding claims is stabilised at temperatures between 100 and 600° C. and is carbonised above 800° C. under inert conditions.

In a preferred embodiment, the carbon fibre is subjected to water vapour, after the carbonisation, at temperatures >200° C., preferably >300° C.

In addition, the present invention provides a carbon fibre which can be produced according to the previously described method for the production of the carbon fibre.

The present invention is explained in more detail with reference to the subsequent embodiments and examples without restricting the invention to the represented parameters.

The lignin is preferably mixed with the thread- or fibre-forming polymer and then dissolved in a suitable solvent by agitation or by a kneading process at an elevated temperature. The resulting solution is possibly filtered and then advantageously shaped to form filaments by wet- or air-gap spinning, which can be stretched under different conditions, washed, treated, dried and then wound up as a filament.

The different lignins, such as alkali lignin, lignosulphonate, thiolignin, organosolvlignin or types of lignin from alternative wood digestion processes, which are known to the person skilled in the art, as occur during cellulose production or also mixtures of these can be used. The lignins are washed intensively with water or possibly also with diluted acids up to an ash content of less than 1%.

As fibre-forming polymers, in particular cellulose or also cellulose derivatives, such as cellulose carbamate and cellulose allophanate, are used.

As solvent, for example aliphatic amides, such as DMF or DMAc, DMSO, tertiary amine oxides, preferably aqueous N-methylmorpholine-N-oxide, particular NMMO monohydrate or an ionic liquid, selected from the group consisting of imidazolium compounds, pyridinium compounds, tetraalkylammonium compounds and mixtures hereof, are used.

The spinning solution is preferably produced with agitation or by kneading at a temperature above 60° C., preferably above 80° C. The polymer concentration is adjusted for example to greater than 8%, preferably greater than 10%. The resulting viscous solution can be filtered using normal filtration devices and can be supplied as a homogeneous particle-free solution for intermediate storage before the spinning process.

The shaping of the spinning solution to form fibres or filaments is effected according to the wet-spinning or air-gap spinning method. In the case of the wet-spinning method, the spinning solution is pressed through the holes of a spinning nozzle, spinning nozzles with hole diameters of 50 to 500 μm being used. The extruded thread is solidified in the coagulation bath which consists of water or of a mixture of the polymer solvent and a nonsolvent. The nonsolvent can be preferably water or an aliphatic alcohol with a chain length up to C 8. When applying air-gap spinning, the viscous lignin-containing spinning solution is pressed through the holes of a spinning nozzle and the extruded threads are stretched in the air gap. The preferred nozzle hole diameter is preferably greater than 100 μm and should not exceed 600 μm. The air-gap length is at least 10 mm. The air-stretched thread is then coagulated comparably to wet-spinning.

The thread is stretched in water and/or a mixture of water and the solvent at a temperature greater than 80° C., preferably greater than 90° C. and in hot air and/or water vapour greater than 90° C., preferably greater than 100° C. up to a multiple of its length but by at least 1.1 times. The stretching can be effected before or even after the washing process.

The stretched and washed thread is treated, before the drying or after the drying or before and after the drying, with a spinning oil with an antistatic effect. Drying is effected on heated rollers or also by through-flow drying at temperatures greater than 80° C., preferably greater than 100° C.

The thus produced fibre comprises at least 10% lignin, preferably >20% lignin and one or more fibre-forming polymers, such as cellulose and/or cellulose derivatives, such as cellulose carbamate and/or cellulose allophanate. The fibres produced according to the invention have a tensile strength of at least 10 cN/tex and a modulus greater than 500 cN/tex and can be converted, according to known methods for stabilising and carbonisation, into carbon fibres and also, by a subsequent water vapour treatment, into activated carbon fibres with a high specific surface.

EXAMPLE 1

60 g of a cellulose (DP_(Cuox)=490) which comprises up to 12% hemicellulose are mixed with 29 g air-dried organocell lignin in 500 g 1-butyl-3-methylimidazolium chloride and dissolved in a horizontal kneader at a temperature of 90° C. within 3 h. The resulting black, homogeneous solution is free of fibre residues and has a zero shear viscosity, measured at 80° C., of 578 Pas.

The solution was pressed through a 40-hole spinning nozzle (hole diameter 200 μm) with the help of an extruder stretched with a drawing ratio of 14 in the air gap and the filaments were precipitated in the acetic acid coagulation bath (pH=2.5). Washing of the filaments was effected with distilled water, drying was effected in air. The filaments had a strength of 25 cN/tex, an elongation of 7.6% and also a modulus of elasticity of 1,320 cN/tex. The lignin content was 20.3%.

EXAMPLE 2

75 g of an air-dried chemical cellulose (DP_(Cuox)=560) are mixed with 48 g air-dried kraft lignin in 500 g 1-ethyl-3-methylimidazolium acetate and dissolved in a horizontal kneader at a temperature of 90° C. within 3 h. The resulting black, homogeneous solution is free of fibre residues and has a zero shear viscosity, measured at 100° C., of 374 Pas. The solution was pressed through a 40-hole spinning nozzle (hole diameter 200 μm) with the help of an extruder, stretched with a drawing ratio of 18 in the air-gap and the filaments were precipitated in the aqueous coagulation bath. Washing of the filaments was effected with distilled water, drying was effected in air. The filaments had a strength of 28 cN/tex, an elongation of 9.6% and also a modulus of elasticity of 1,560 cN/tex. The lignin content was 36.4%.

EXAMPLE 3

30 g cellulose carbamate (DP_(Cuox)=258, DS=0.4) are mixed with 10 g air-dried organosolvlignin and 70 g 1-butyl-3-methylimidazolium acetate and dissolved in a horizontal kneader at a temperature of 110° C. within 2 h. The resulting black, homogeneous solution is free of fibre residues and has a zero shear viscosity, measured at 100° C., of 1,215 Pas. The solution was pressed through a 12-hole spinning nozzle (hole diameter 150 μm) with the help of an extruder, stretched with a drawing ratio of 30 in the air gap and the lignin-containing cellulose carbamate fibre was precipitated in the aqueous coagulation bath with 15% 1-butyl-3-methylimidazolium acetate. Washing of the filaments was effected with distilled water, drying was effected in air. The filaments had a strength of 45 cN/tex, an elongation of 6.4% and also a modulus of elasticity of 2,346 cN/tex. The lignin content was 18.3%.

EXAMPLE 4

447 g of an air-dried chemical cellulose (DP_(Cuox)=560) are mixed with 276 g air-dried kraft lignin and also 2.1 g propylgallate in 5.2 kg 52% N-methylmorpholine-N-oxide hydrate and dissolved in a horizontal kneader at a temperature of 90° C. with distillative water separation within 3 h. The resulting black, homogeneous solution is free of fibre residues and has a zero shear viscosity, measured at 90° C., of 642 Pas. The solution was pressed through a 40-hole spinning nozzle (hole diameter 200 pm) with the help of an extruder, stretched with a drawing ratio of 20 in the air gap and the filaments were precipitated in the aqueous coagulation bath. Washing of the filaments was effected with distilled water, drying was effected in air. The filaments had a strength of 42 cN/tex, an elongation of 5.4% and also a modulus of elasticity of 2,164 cN/tex. The lignin content was 52.4%. 

1. A method for the production of a lignin-containing precursor fibre for the production of carbon fibres and/or activated carbon fibres, comprising a) forming a solution, comprising at least one sort of lignin and also at least one fibre-forming polymer selected from the group consisting of cellulose or cellulose derivatives in at least one solvent, selected from the group consisting of tertiary amine oxides, ionic liquids, aprotic polar solvents, dimethylformamide and dimethylacetamide, and mixtures thereof, and b) transferring the solution into a coagulation bath by extruding the solution through a spinning nozzle by a wet spinning or an air-gap spinning method, the lignin-containing precursor fibre precipitating.
 2. The method according to claim 1, wherein the solution formed in step a) is produced by agitation or kneading of the at least one sort of lignin and also of the at least one fibre-forming polymer in the at least one solvent, at temperatures of 60° C. or more.
 3. The method according claim 1, wherein the solution is filtered before introduction into the coagulation bath.
 4. The method according to claim 1 wherein the spinning nozzle has a spinning hole diameter from 50 to 600 pm.
 5. The method according to claim 4 wherein the transferring step is performed by the wet spinning or the air-gap spinning method, wherein there is an air gap of at least 10 mm and at most 500 mm.
 6. The method according to claim 1 wherein the lignin-containing precursor fibre a) is stretched to at least 1.1 times its length, at a temperature of at least 60° C., the stretching being implemented preferably in the precipitation bath, in air or in water vapour, b) is washed with demineralised water, c) is treated subsequently with textile aids for improving the thread strength and for avoiding electrostatic charges, d) is dried by winding up or winding round of the fibre onto or around heated rollers or by through-flow drying at a temperature of at least 80° C., and e) is wound up.
 7. The method according to claim 6 wherein the fibre is treated with a spinning oil before drying, after drying or before and after drying.
 8. The method according to claim 1 wherein respectively relative to the entire solution, a) the at least one sort of lignin has a concentration from 2 to 30% by weight, and b) the at least one fibre-forming polymer has a concentration from 5 to 40% by weight.
 9. The method according to claim 1 wherein the coagulation bath comprises water or a mixture of water and an organic liquid selected from the group consisting of aprotic polar solvents, aliphatic amides which are liquid at room temperature, tertiary amine oxides, and ionic liquids.
 10. The method according to claim 9 wherein the pH value of the coagulation bath is between 1 and
 7. 11. The method according to claim 1 wherein the aprotic polar solvent is dimethylsulfoxid (DMSO) and the tertiary amine oxide is aqueous N-methylmorpholine-N-oxide (NMMO), and the ionic liquids are selected from the group consisting of imidazolium compounds, pyridinium compounds and tetraalkylammonium compounds, and mixtures thereof.
 12. The method according to claim 1 wherein the cellulose or cellulose derivatives, are selected of the group consisting of cellulose carbamate, cellulose allophanate, hemicelluloses, and mixtures thereof.
 13. The method according to claim 1 wherein the at least one sort of lignin results from wood and annual plant pulping methods and is selected from the group consisting of alkali lignin, kraft lignin, lignosulphonate, thiolignin, organosolvlignin, ASAM lignin, lignins from digestion processes by means of ionic liquids or enzymes and combinations thereof.
 14. A precursor fibre for the production of carbon fibres, comprising: a) a content of at least one sort of lignin of 1% to 99% weight, b) a content of at least one sort of fibre-forming polymer of 1 to 99% by weight, and c) a strength of at least 10 cN/tex, or a modulus of elasticity of at least 1,000 cN/tex.
 15. The precursor fibre, produced according to a method according to one of the claim
 1. 16. A method for the production of a carbon fibre in which a precursor fibre according to claim 14 is stabilised at temperatures between 100 and 600° C. and is carbonised above 800° C. under inert conditions.
 17. The method according to claim 16 wherein the carbon fibre is subjected to water vapour, after the carbonisation, at temperatures greater than 200° C.
 18. A carbon fibre, producible according to a method of claim
 1. 19. The method according to claim 2 wherein the temperature is above 80° C.
 20. The method according to claim 4 wherein the spinning hole diameter of the spinning nozzle is between 100 and 500 pm.
 21. The method according to claim 5 wherein the air gap is at least 20 mm.
 22. The method according to claim 6 wherein the lignin-containing fibre is stretched to at most 12 times.
 23. The method according to claim 6 wherein the lignin-containing fibre is stretched to at least 1.5 times.
 24. The method according to claim 6 wherein the lignin-containing fibre is stretched at a temperature of at least 80° C.
 25. The method according to claim 6 wherein the lignin-containing fibre is stretched at a temperature of at least 100° C.
 26. The method according to claim 6 wherein the lignin-containing fibre is dried at a temperature of at least 100° C.
 27. The method according to claim 8, where the concentration of the at least one sort of lignin is between 3 and 20% by weight.
 28. The method according to claim 8 wherein the concentration of the at least one fibre-forming polymer is between 7 and 30% by weight.
 27. The method according to claim 9 wherein the ionic liquids are selected from the group consisting of 1 -butyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium acetate, and 1-ethyl-3-methylimidazolium acetate.
 28. The method according to claim 10, wherein the pH value of the coagulation bath is between 2 and
 5. 29. The precursor fibre according to claim 14 wherein the fibre has a strength of at least 20 cN/tex and a modulus of elasticity of at least 1,300 cN/tex,
 30. The method according to claim 17, wherein the carbon fibre is subjected to water vapour, after the carbonisation, at temperatures greater than 300° C. 